The invention relates to an apparatus for the separation of suspended matter from a fluid under pressure.
Suspended matter are particles typically in a size range between one thousand of a millimeter and some millimeters, and can be present in concentrations of a few milligrams up to a few grams per liter of fluid.
It is known to separate such suspensions in a bed in which granular material is used as the filter medium. As a particularly cheap and at the same time sufficiently effective, granular material, broken or ground (rounded) sand, e.g., quartzsand, as well as ground anthracite have proven themselves. These granulates are relatively hard and do not swell during use. The invention takes also other not swellable granulates into consideration, such as e.g., small silicate balls, small hollow balls of aluminum oxide as well as other manmade granulates of comparable kind, inclusive of plastic or epoxy material, which have a lower hardness than, for instance, sand, but do not really swell under the filtration conditions. Preference is given to such granular mediums which behave principally inertly during the filtration process. It is also possible to apply granulates of various grain sizes, as well as mixtures of different kinds of granulates.
Granulate beds have the advantage that by very simple means, that is, by using an adequate size of granulate, almost any required size of pore can be reached in order to undertake in these pores, in principal and apart from some surface and other effects of second order, all the wanted mechanical separation.
It is known (DE-AS No. 11.25.889) to use stratified granulate beds, in which the grain size decreases in the direction of flow of the fluid to be filtered. In many cases, however, also homogeneously mixed granulates of various grain size are used. Granulate beds of this kind do not require a regeneration like some beds of activated carbon. They must, however, be freed from separated matter by means of a backwash flow in the reverse flow direction. With conventional equipment of this kind, the granulate bed normally is flowed through in one direction by the fluid to be filtered for such a period of time until, owing to separation of an increasing quantity of suspension, the pressure drop over the filter bed reaches a critical limit. Then the granulate bed is purified by means of a wash fluid in the reverse direction of flow to remove the separated suspensions via a separate outlet.
The most usual granulate beds are those in which, prior to processing, granulates of various particle sizes are arranged in such a way that the fineness of the particle size proceeds in the downward direction. Such granulate beds are then downwardly perpetrated with the fluid to be filtered until a backwash in the reverse direction will be required. This backwash flow, however, fluidizes the granulate bed and by doing so it causes a mixing of all the particles present in the bed, so that after some time, contrary to the required division of pores, the granulates of finer size lie at the top and the granulates of larger size at the bottom.
It is already been tried to avoid this inversion-effect by using granulates of different specific gravity, but without reaching convincing results in preventing the above described mixing. Besides, granulates that are selected according to their specific gravity are not always the optimum for the required filtration-function.
With granulate beds for the filtration of a liquid fluid, often additional fine-divided air is supplemented during the backwash procedure in order to transfer the granulate bed into a fluid bed, so that in this way separated suspensions can still better be freed. One then definitely runs the risk of carrying granulates of the finer size particles along with the backwash fluid through its outlet. In order to prevent this, it has already been considered to keep the granulate bed together by means of a collecting device for the fine grained granulate, e.g., a screen on top of it. (DE-PS 832 596)
As an alternative it has also been considered to have the granulate bed flowed through by the fluid to be filtered in an upward direction. However, this causes even greater disadvantages. Initially, the filtrate can already remove fine size grained granulate from the granulate bed, when a collecting device, e.g., a screen, is not mounted over the granulate bed. Furthermore, this alternative can lead to clod-formation of the separated suspended solids with the fine grained granulate and such clod-formation can only very difficultly be washed out of the granulate bed. Furthermore it is practically impossible to fluidize the granulate bed in reverse direction, viz. in a downwards direction. Consequently, when flowing a fluid from bottom to top through the granulate bed, the backwash fluid also flows from bottom to top through the granulate bed, which results in difficulties with the purification (see: "Waste Water Engineering, Metcalf Eddy Inc., 2nd Edition, McGraw Hill Company, New York, page 229-231, especially page 230, FIGS. 6-30, especially FIG. 3B").
With vertical flown through conventional granulate beds, a very thick layer was thought to be required, to prolongate the operation time of the separation process and hence to use relatively seldom backwashing for purification. Nevertheless, the filtration had to be interrupted for backwashing, either every day or at least in a period of a few days, since as a matter of fact that the real active separation layer represents only a marginal part of the total thickness of such a flowed through granulate bed and this separation layer, in which fine grained granulate is deposited during inversion, is relatively often plugged-up. A construction is known in which granulates are filled in the intervening space between two concentric vertical cylinders and a fluid to be filtrated conducted radially and consequently horizontally through this cylinder-shaped granulate bed from the outside to the inside (HU-PS No. 172 006). In this process it can, however, come to a seperation in the granulate bed during the filtration process, due to flowing downwards of coarse grained granulates and flowing upwards of fine grained granulates. It is also possible that, due to setting of the granulate, openings will occur in the cylindric layer of granulates resulting in a free flow of fluid without separation. With the known similar apparatus (DE-OS No. 2136887), which is especially destined for the filtration of water, e.g., waste water, various vertically positioned granulate beds are introduced, especially fine sand, with a grain size spectrum of 0.05 to 0.2 mm, which are pressed in the recesses of the lattice-like supports with a precompression of at least 10%, preferably more, referred to the normal total density, and held in place by means of porous layers of gauze, which in turn are held by ribs of a distance support. In this process, the porous layer, however, beats out under the pressure of expansion of the pre-compressed granulates. The granulate composition should be as much as possible of the same grain size, so that a filtration effect in essence takes place at the surface of the filter elements exposed to the filtrate and due to this, the rest of the granulate layer--down stream from the surface--has practically no effects on the separation capacity. The known apparatus can only be manufactured in a relatively complicated way, cannot simply be transferred on sand beds with a coarser size of granulate, and is not only limited in the degree of effects due to the essentially only superficial filtration effect, but besides is only suitable for liquids with small suspension load, since otherwise the required backwash frequency will become too high.
Both aforementioned known apparatuses according to HU-PS No. 172006 and DE-OS No. 21 36 877 already underlie the aim, to provide a granulate bed with a comparatively thin layer of granulates, however, without fully succeeding from the point of proper functioning due to the aforementioned reasons.